How can I switch back to an older version (stable) of the software?
You must know the number of the older version you want to step back to (e.g.: 0.4.0-b373) :
Open a console (or remote ssh-console).
sudo apt-get install fabscanpi-server=0.4.0-b373
Follow the questions in the installation procedure and finally restart the FabScanPi again.
NOTE: Please make sure you selected the correct packet source in the